Mary Gregory to Ed Humpherson: Mid-year population estimates for England and Wales

Dear Ed,

Mid-year population estimates for England and Wales

I am writing to you today to acknowledge the mid-year population estimates (MYE) compliance review report you published in May. We would like to thank you for your engagement with my teams during this process and for the findings of the review. The report confirmed that the continued accreditation of these statistics is conditional on ONS meeting four requirements before or at the same time as the next publication of the MYEs, in summer 2026.

We have today published the mid-2025 population estimates for England and Wales, which include revisions to mid-2022, mid-2023 and mid-2024 estimates to incorporate the improved long-term international migration estimates. We have also published a blog and a suite of supporting information to help users of population estimates understand the quality of the estimates and what this means for how they can be used. In delivering this work, we have tried to take a comprehensive and user-focused approach to addressing the requirements in the compliance review, while improving the overall experience for users of the estimates.

The publication of MYEs includes a statistical bulletin which contains clearer and more prominent information about the uncertainty around the estimates and guidance on how the estimates can and cannot be used. Alongside the publication we have also published additional articles – linked below – which provide detailed information on the strengths and limitations of the MYEs, to aid users’ confidence in them and increase understanding of how they can be used appropriately. The article about the effect of the coronavirus pandemic on data and quality of population estimates outlines the effect the pandemic had on different components of the MYEs and how these have been managed. The publication of the MYEs contains the following components:

The article about improving population estimates covers the areas we will focus on over the next two years: data sourcing, supply and quality, as well as dissemination of outputs and user engagement. In particular, our work to improve the methods and data used for producing subnational estimates of international migration and estimates of internal migration will be central to the improvements to MYEs. Our aim is to ensure we have a coherent system for producing population statistics which supports user confidence. The work identified in this plan will also help us continue to progress the recommendations outlined in the MYE compliance review.

We will continue to work closely with local authorities and other users of population estimates to better understand local population change and the challenges associated with measuring it. This collaboration will help us improve the quality of the estimates, reduce uncertainty over time, and provide greater confidence in areas where population change is more complex, including those with large student populations, high migration and mobile working-age populations.

We will publish updates on progress within the population and migration quarterly update, along with providing an additional layer of assurance by providing an update to the Population Statistics System Committee every six months. We will also continue to deliver improvements to our critical outputs in the near term through a Population Statistics System Plan to be published later in 2026. We welcome your continued engagement as this work progresses.

Yours sincerely,

Mary Gregory

Annex: Requirements in the compliance check of mid-year population estimates for England and Wales, and ONS’ actions completed and planned

Requirement 1

To support user confidence in its decision to continue with the MYEs, ONS must be open and explicit about the strengths and limitations of the current methods and what action ONS is taking, with time scales, to address methodological limitations with independent assurance.

Action:

a) ONS has introduced a new Quality and Methods Guide (QMG) to describe the strengths and limitations in more detail.

b) The MYE statistical bulletin is more explicit in describing what can and can’t be done with the estimates and highlights cautionary notes more clearly than in previous bulletins.

c) ONS has published a mid-year estimates improvement plan alongside the MYEs to outline how they will address limitations and improve the quality and reliability of the MYEs.

d) A blog to pull everything together and make clear what this means for users of population estimates.

e) Ensure understanding for users through stakeholder engagement in autumn 2026, and written information

Requirement 2

To ensure that users have confidence in the population baseline underpinning the MYEs, ONS must:

Clearly set out the pandemic’s impacts on the MYEs, prominently linking to its published COVID-19 impacts report, and publish evidence to justify its claim that its estimates since 2021 are unaffected by systemic pandemic effects, particularly in light of acknowledged uncertainties for some areas.
Actively engage with users through its established groups to test the adequacy of current guidance and make necessary improvements, ensuring users are explicitly informed about which data and geographies remain vulnerable to pandemic-related distortions

Action:

a) Published an article which describes the pandemic impacts on MYE

b) Published the Local authority quality of population estimates for England and Wales and Improving population estimates for England and Wales. Engaged with Ceredigion, Cambridge, Cambridgeshire, the Greater London Authority, the Department for Education and the Royal Statistical Society before publication, and will further engage with a wider range of LAs in Autumn

c) Updated the signposting on the previously published report about the effects of the pandemic on Census 2021, to inform readers of the newly published article Effect of the coronavirus pandemic on the quality of mid-year population estimates for England and Wales

d) Published a blog to pull everything together and make clear what this means for users

e) Will seek feedback on the published materials which accompany the MYEs from users of them, to ascertain their understanding of the guidance and identify further improvements.

Requirement 3

To help users better understand the strengths and limitations of the MYEs, ONS must:

Improve the prominence and clarity of how it communicates uncertainty in its population estimates, particularly for local authority areas with high levels of migration and/or large student populations. This should include enhancing communication for non-technical users so that they can understand what the estimates should and should not be used for.
Improve the accessibility, structure, and navigation of key methodological and quality and methodology information (QMI) documentation to support a broader range of users.

Action:

a) Published an article on the Local authority quality of population estimates for England and Wales

b) Reviewed and updated methods materials which accompany the MYEs and introduced a Quality & Methods Guide.

c) The MYE statistical bulletin includes confidence intervals in the analysis highlights the existence of uncertainty around the estimates.

d) We will engage with a broad range of users on QMG documentation and other technical guidance to ensure user understanding

e) Published a blog to pull everything together and make clear what this means for users

Requirement 4

To avoid the risk of users picking up the wrong number for the purpose that they need, ONS must update and promote its guidance for users on which population estimates should be used for which purposes, setting out the strengths and limitations of each.

Action:

ONS has updated the Population Statistics and Sources guide, which explains the uses of the different population estimates in existence.